(1) Field of the Invention
This invention relates to a thermosetting powder resin based water-slurry coating composition, and particularly relates to a thermosetting powder resin based water-slurry coating composition (hereinafter referred to as a water-slurry coating composition) improved in the anti-sedimentation properties and redispersibility of particles in the slurry and capable of forming a film having excellent smoothness, gloss, and other physical and chemical properties such as water resistance, moisture resistance, corrosion resistance, and the like without developing any checking, and phenomena of foaming in a coating of the slurry mainly due to a sudden evaporation of a dispersing medium and of developing pinholes, and craters in a film due to foaming, which phenomena are hereinafter referred to simply as pinholing and cratering by solvent popping, on film formation by heating.
(2) Description of the Prior Art
A great demand for the elimination of environmental pollution as well as saving of resources in the field of coatings and coating compositions has recently arisen, and water-slurry coating compositions have been developed for practical use in order to meet the demand.
For example, U.S. Pat. No. 3,787,230 corresponding to Japanese Patent Laid-Open Publication No. 45529/1973 discloses a method of applying a powder paint to an article to be coated by forming a slurry of uniformly suspended non-water soluble powder paint in water, and also discloses a wide variety of film formers such as acrylic resins, alkyds, vinyls, polyesters, nylons, epoxy resins, and the like as the basis for the powder paint. The water-suspended resin slurry as above is formed by dispersing a resin powder having a particle size normally ranging from 1 to 10 microns in a water-based medium at a solids content of from 10 to 60 percent by weight, so that the above formulation has such advantages that the coating procedure for the conventional liquid paint is available without any environmental pollution and toxicity due to organic solvents used, and that a finish with a relatively thick film can be obtained by a single coating procedure.
The water-suspended resin slurry has further such advantages that the film formed therefrom has good water resistance, moisture resistance, chemical resistance, etc. because a surface active agent is not used in large amounts as an ingredient thereof.
It should be pointed out, however, that the water-dispersed resin slurry with advantages as above has the following drawbacks due to its own property.
The typical drawback is in that a relatively large amount of watersoluble thickening agent, an inorganic pigment dispersion including an extender pigment, a water soluble resin, etc., must be used.
Japanese Patent Publications Nos. 4149/1980 and 4150/1980 disclose aqueous dispersion coating compositions in which an aqueous resin or an aqueous resin having an auxiliary chemically reactive group is incorporated. These aqueous dispersion coating compositions are, however, unsatisfactory in connection with water resistance, and moisture resistance, and the coexistence of the aqueous resin therewith results in sedimentation of resin particles in the water dispersion system to cause poor dispersion stability.
Further, Japanese Patent Publication No. 4342/1980 discloses a water-dispersed coating composition in which a hydrophilic ultrafine inorganic compound, that is, the aforesaid inorganic pigment is incorporated in the water dispersion system. The coating composition, however, inevitably has such a drawback that the mineral pigment inhibits the flow characteristic on melting of resin on film formation by heating, and consequently reduces the smoothness, gloss, etc., of the film. The coating composition further has such a drawback that pinholing and cratering by solvent popping, and checking are liable to develop in the film on film formation by heating.
In order to overcome the problems described above, Japanese Patent Laid-Open Publication No. 132630/1979, for example, discloses a water-dispersed thermosetting resin coating composition in which polyethylene glycol having an average molecular weight of from 200 to 10,000 is added to a special thermosetting resin powder. The film formed therefrom is inevitably degraded in water resistance and moisture resistance. Moreover, the coating composition is unsatisfactory in connection with the anti-sedimentation properties and redispersibility of particles in the slurry coating composition, and consequently with the extended storage stability of the coating composition.
As a characteristic example in the case where a water soluble resin is incorporated, Japanese Patent Publication No. 433/1980 discloses an aluminum flake pigmented slurry coating composition. According to the coating composition, an intention to obtain a film with good brightness may be achieved, but the coating composition apparently has drawbacks described above.
For the purpose of overcoming problems particular to the slurry type coating composition in connection with the application and preparation thereof, Japanese Patent Laid-Open Publication No. 144442/1979 discloses a method of applying a slurry paint which comprises heating a substrate beforehand so that its surface temperature may be 50.degree. C. or higher, applying the slurry paint thereon at prescribed surface temperature, and heating to a temperature higher than the melting point or softening point of the film forming material in the slurry paint to form a film, and Japanese Patent Publication No. 4341/1980 discloses a process for the preparation of a slurry type water-dispersed coating composition which comprises mixing a paint component containing resin as the major ingredient to form a pellet-form composition, adding water and a surface active agent to the composition to reduce the surface tension of the resulting mixed solution to less than 35 dyne/cm, and grinding the composition in water to disperse the paint component in the liquid. Further, U.S. Pat. No. 4,100,315 discloses a process for the production of a thin, homogeneous, adhesive, non-porous film of lacquer on a metal surface an aqueous composition suspension having extended storage stability and comprising a solid thermosetting synthetic resin fine powder and a wetting agent selected from a special alkylphenyloxyethylate and a special amino-coconut fatty acid oxyethylate, drying, and curing the composition. These processes described above, however, teach nothing about the subject matter of the present invention which consists in solving simultaneously the problems in connection with the stability of dispersed resin particles in the dispersion such as anti-sedimentation properties and redispersibility of particles; the flow characteristic on melting of resin on film formation by heating such as the development of pinholing and cratering by solvent popping, and checking; physical and chemical properties of a cured film such as gloss, smoothness, water resistance, moisture resistance, and corrosion resistance; and the like.